The present invention relates to a fused plug, and more particularly, to such a plug constituting a safety lamp plug, and intended for providing fusing of light string sets.
Light string sets of the type utilized for decorative purposes, and particularly the type having miniaturized incandescent lamps, typically are manufactured with relatively small gauge insulated wire. For protecting such string sets against short circuits and to provide the sets adequately with intrinsic safety for permitting their approval by safety rating authorities or organizations, as well as to comply with various electrical codes, it is desirable to provide fusing for the sets. It has heretofore been proposed to utilize various conventional fuses in various ways as a part of the light string set, including placing fuse lamps within the end sockets of a series circuit string for protecting lamps between the ends of the string. Such an arrangement is disclosed in U.S. Patent Application Ser. No. 939,974, now U.S. Pat. No. 4,223,248, of George Kwo Kiun Tong, entitled "Fused Light String Set".
It has also been proposed to incorporate conventional cartridge fuses with the body of the plug of such a set. However, certain operational and economic disadvantages have resulted from such configurations. For example, cartridge fuses have required metallic contacts in the plug at each end of the fuse which add to the cost and the associated labor in manufacturing the plug. Furthermore, conventional cartridge fuses have been known to explode when a high current passes through them, such as may result from a short circuit. Also, they may be subject to variation in the rating tolerances so that an element of doubt exists as to the precise current at which a cartridge fuse will interrupt the circuit. Further, the design of conventional plugs in which cartridge fuses are placed side by side in a single compartment may compromise safety since the explosion of one cartridge fuse may damage the other and present the possibility of adjacent electrodes coming into contact. Another disadvantage of previous designs is that the prongs of the plug may be unreliably retained, presenting the possibility of the plug becoming broken or pulled apart during normal use. Also, some plugs employing cartridge fuses have provided unreliable electrical contact resulting in failure or intermittent operation, particularly when the plug has been dropped on a hard surface or has been exposed to weather and to temperature changes.
Another difficulty which has been encountered with prior fused plugs employing cartridge fuses is that it is difficult to design the plug in such a way that a cartridge fuse in which the fusible element has been melted by an overload or short circuit can be replaced by the consumer. There is such a risk that the consumer may replace a cartridge fuse with one of an incorrect rating that, for purposes of safety, the fused plug may be required to be sealed during manufacture. This prevent replacement of cartridge fuse in the plug. Thus, if a sealed plug incorporating fuse cartridges has operated to interrupt the circuit by melting of the fusible elements of the fuses therein, the plug must be entirely replaced or the light string set discarded. Therefore, it is desirable to design the light string set and plug in such a way that the consumer can safely replace the fuses without having to discard the plug or light string set and can easily determine which fuse requires replacement.
It is an object of the present invention to provide an improved fused plug, and more particularly, such an improved plug for use with light string set.
It is another object of the present invention to provide such an improved fused plug which incorporates fused lamps or so-called safety lamps, whereby the fused plug constitutes a safety lamp plug, and more particularly, which incorporates miniature glass-enveloped fuse lamps.
It is a further object of the present invention to provide such a fused plug incorporating fuse lamps of the character stated which are economical, which have high accuracy, and exhibit precise tolerance in operation.
It is a further object of the present invention to provide such a fused plug which is of compact configuration and small dimensions, and which operates to provide uniform, accurate, electrically reliable fusing of light string sets or like electrical loads.
A still further object of the invention is the provision of such a fused plug which is not prone to failure or intermittent operation resulting from vibration, droppage, weather exposure, temperature changes or other hard conditions of use.
Another object of the present invention is the provision of such a fused plug which is physically superior and not prone to being damaged or pulled apart during normal use.
Yet another object of the invention is the provision of such a fused plug incorporating fused lamps which lamps can be quickly and easily replaced by the consumer and which plug, in addition, permits the consumer to be provided with a clear indication of proper operation of the fused lamps.
Among other objects of the invention may be noted the provision of such a fused plug which has high intrinsic safety yet can be manufactured very economically and reliably; and which in general provides a very high degree of safety for the consumer.
Other objects and features will be in part apparent and in part pointed out in the following description.